4. Optical position sensor, photo-current measurement

Transcription

1 4. Optical position sensor, photo-current measurement Task 1. Measure the output signal of photo-electric sensors Measure the relative dependency of output signal versus light intensity. The dependence of produced light intensity versus excitation current is supposed to be linear (for the LED used to excite the photo-electric detectors). Prepare charts showing the output current dependency on light intensity, each containing at least six different current values: a) photodiode BPW34 (Vishay) in short-circuiting connection, b) photo-transistor PT204-6C (Everlight) 2. Dynamic behavior of a photo-transistor based sensor Determine the output response of a PT204-6C photo-transistor on optical impulse. Estimate maximum frequency of a rectangular optical signal that could be observed with that phototransistor. The light source is excited by impulses 0.5 ms long with a frequency f=300 Hz (impulse generator G5-6). Observe the excitation as well as the output signal with a digital oscilloscope. Make a chart of those signals. What is the rise and fall time of the input and output waveform? 3. Reflexive sensor with a diffuse reflection Observe the properties of a simple reflexive sensor. The sensor consists of two elements. Red LED illuminates the target and the photo-transistor PT204-6C senses the reflected light. The target is a piece of a white office paper (100 x 80 mm). What is the main problem of this simple sensor, how the ambient light influences the measurement? Measure the photo-transistor response if the LED is permanently switched off (on). Make a chart: photo-transistor output versus the distance of the target (distances 5, 10, 15 and 20 cm). The LED is switched on if the MOD input is left unconnected and switched off if the MOD input is tied to ground. Up to which distance is there a change in output between the two states (LED on - off)? 4. Suppression of the ambient light influence by modulation Use modulation of the LED excitation current in order to eliminate the influence of ambient light on the sensor output. Use the pulse generator set to approximately 300Hz for the LED excitation. Check the output signal with the oscilloscope. Insert a high-pass filter between the sensor output and the oscilloscope. The filter should remove the DC part of the signal and greatly reduce the low frequency disturbances caused by fluorescent lamps. Observe the amplitude of the impulses at the high-pass filter output with respect to the distance of the reflective target. 5. Demonstration of the functionality of the LEGO NXT optical sensor Check out the functionality of the LEGO optical sensor as a target proximity detector. Use the same target as in previous tasks (100x80mm white paper card). There are three modes programmed to the NXT control unit: LED on, LED off, LED blinking. Observe the ambient light influence on the ability to detect the target in different modes. How to run the NXT application: connect the NXT brick to the power supply (12VDC), switch on by pressing the orange button, select and run program called SME by repeated pressing of the orange button. Program & sensor functionality: 1/8

2 1. Measures the ambient light (LED switched off) 2. Measures the ambient light + the reflected light (LED switched on) 3. LED is blinking at moderate frequency (the frequency is limited by the software...) Three values are presented on the brick s LCD: OFF (ambient light), ON (ambient light + reflected light), DIFF (difference between ON and OFF) The external sensor-switch located on the right side of the brick is used to change the modes. Notes and explanations -> 1) Measurement of the photo-electric detectors output signals There are two LEDs connected in series which illuminate the pair of sensors: photo-diode FD BPW34 and photo-transistor FT type PT204-6C. Overall simplified circuit diagram is presented on Fig.1. Symmetrical power supply is needed to power up the circuit: +12 V a -12 V, e.g. (or possibly +15 V a -15 V e.g. BK125). 0 () 100 k U K + ma In i A 820 regul. power supply + Gnd FD voltmetr Hi Com 1234 K FT Fig.1 Overall circuit diagram for measurements with the photo-diode and photo-transistor The photo-diode FD is connected to the terminal marked. The measurement is done in a short-circuited mode (terminals and In are interconnected). There is a current to voltage converter with op-amp OP482. The photo-transistor is connected like an emitter follower. The parasitic capacitance of the drain-gate junction then less influences the dynamic properties. The resistor connected to emitter (10 kω) serves as a current to voltage converter ( ). The maximal current going through the LEDs (measured by ammeter) should not be higher than ma. The current sufficient for the measurement is about 1 to 5 ma. Current calculation: LED forward voltage is approximately 2V, there are two LEDs connected in series. 2/8

3 -> 2) Measurement of dynamic properties of the photo-transistor Use an impulse generator to generate current pulses for the LED excitation. terminal of the generator is connected to K input; output of the generator is connected to A input. There are two LEDs in series, it is needed to provide at least 4V (high level of the output pulse) to light up the LEDs (check the level with an oscilloscope). 0 () 100 k U K In i A 820 oscil. impuls gen. Out Gnd FD CH1 CH2 K FT Fig.2. Circuit for measurement of dynamic properties of the photo elements -> 3) Reflective sensor with diffuse light reflection The circuit is powered through linear voltage regulator so it accepts input voltage between V. The LED is controlled via modulation input MOD. The LED is switched on if the input is not connected; switched off if the MOD input is grounded Ucc = +5 V 1n5 330k 2k7 240 voltmetr Hi Lo 1234 MOD 27 k Fig.3 Overall circuit diagram of the reflective sensor (static measurement) 3/8

4 -> 4) Suppression of the ambient light influence by modulation The MOD input is excited by the square wave from the pulse generator (low level = 0V, high level = 3...5V). Set the pulse length to 0.5ms and frequency to 333Hz. Use the same signal for the synchronization of the oscilloscope (trigger on this channel). Compare the signal before and after the high-pass filter ( and terminals) Ucc = +5 V 1n5 330k oscil. 2k7 240 CH1 CH2 impuls gen. Out Gnd MOD 27 k Fig.4 Overall circuit diagram of the reflective sensor (dynamic measurement) -> 5) Sensor LEGO NXT a) There is a raw ADC value shown on the LCD (0-1023), it corresponds to 10-bit AD converter output of the ATMEGA48 microcontroller. It is given by the electric circuit that lower value means higher illumination of the sensor (photo-transistor). b) In order to make the output values more stable the ADC readings are averaged (256x in static mode, 8x while blinking). It suppresses the disturbance caused by fluorescent lighting (100Hz) and eliminates the electric noise effects (CPU clock, switched power supply...). Fig. 5 Circuit diagram of the LEGO MINDSTORMS NXT light sensor 4/8

5 LEGO NXT brick source code (Bricx), uses light sensor and one button sensor to change the operational mode: // Senzory a mereni - light sensor demo // unsigned char state = 0; unsigned char ready_count = 0; bool ready_flag = false; bool blink_flag = false; long avg; long avg_0; long avg_1; int i; string msg; #define TOUCH_WAIT 40 task main(){ TextOut(0, LCD_LINE3, "LED OFF"); TextOut(0, LCD_LINE5, "out:"); SetSensorLight(IN_1,false); SetSensorTouch(IN_2); while(1){ //init if(sensor_2 && ready_flag){ state++; if(state > 2){state = 0;}; ready_flag = 0; ready_count = 0; switch(state){ case 0: blink_flag = false; Wait(10); TextOut(0, LCD_LINE3, "LED OFF"); TextOut(0, LCD_LINE5, "out:"); SetSensorLight(IN_1,false); break; case 1: blink_flag = false; Wait(10); TextOut(0, LCD_LINE3, "LED ON"); TextOut(0, LCD_LINE5, "out:"); SetSensorLight(IN_1,true); break; case 2: Wait(10); 5/8

6 TextOut(0, LCD_LINE3, "LED BLINKING"); TextOut(0, LCD_LINE5, "out OFF:"); TextOut(0, LCD_LINE6, "out ON:"); TextOut(0, LCD_LINE7, "out DIFF:"); blink_flag = true; break; } }//if(sensor_2 && ready_flag){ ready_count++; if (ready_count > TOUCH_WAIT){ ready_flag = true; ready_count = 0; }; //(ready_count > TOUCH_WAIT) if (blink_flag){ avg_0=0; avg_1=1; for(i=0;i<8;i++){ //filtering ready_count +=3; SetSensorLight(IN_1,true); Wait(4); avg_1 += SensorRaw(S1); Wait(1); SetSensorLight(IN_1,false); Wait(4); avg_0 += SensorRaw(S1); Wait(1); }//for(i=0;i<8;i++) avg_0 /=8; avg_1 /=8; sprintf(msg, "%+04d", (avg_0)); TextOut(60, LCD_LINE5, msg); //LED OFF sprintf(msg, "%+04d", (avg_1)); TextOut(60, LCD_LINE6, msg); //LED ON sprintf(msg, "%+04d", (avg_0 - avg_1)); TextOut(60, LCD_LINE7, msg); //DIFFERENCE }else{ //!(blink_flag) avg=0; for(i=0;i<256;i++){ //filtering avg += SensorRaw(S1); }//for(i=0;i<256;i++) avg /=256; sprintf(msg, "%+04d", (avg)); TextOut(30, LCD_LINE5, msg); }; //(blink_flag) }//while(1) }//main() 6/8